We have studied the potential for KIR3DS1 interactions with HLA Bw4 80I. Surprisingly, HLA tetramers folded with a variety of HIV-derived peptides did not interact with KIR3DS1. The inhibitory allele of KIR3DS1, KIR3DL1, binds Bw4 in a peptide-dependent manner suggesting that KIR3DS1 binding may require identification of the appropriate peptide. Most importantly, we have demonstrated for the first time KIR3DS1 expression on a substantial portion of peripheral NK cells through its recognition by the monoclonal antibody Z27. In donors homozygous for KIR3DS1 as many as 70% of NK cells express the receptor. Moreover, we have found that KIR3DS1 can be expressed on the NK/T cells of some individuals suggesting the receptor can regulate that population as well. In addition, to defining its expression, we have now shown that ligation of KIR3DS1 by Z27 leads to NK cell interferon gamma production and degranulation. Furthermore, we have documented the persistence of KIR3DS1+ NK cells in HIV viremic patients. The high frequency of KIR3DS1 expression, together with its ability to activate NK cells and its maintenance during HIV viremia, strongly support the epidemiological data suggesting a critical role for this KIR in HIV pathogenesis.